The present invention relates to a protected electronic mass memory unit, the memorizing elements of which comprise semiconductor memories, for instance of the RAM or EEPROM type. More particularly, it is intended to be used as a mass memory subsystem in an information processing system.
An information processing system is known to constitute a central processing unit, embodied by at least one central processor and one main memory to which this processor is connected, various peripheral devices, and an input/output processor assuring the control of the exchange of data between the memory and these various devices.
Peripheral controllers are associated with the various peripheral devices and assure the physical transfer of the data between the central processing unit and the peripherals associated with these various controllers.
All these functional constituent elements of an information processing system are arranged on a set of boards, generally connected to the same parallel bus, that assures the transport of data between the various boards and the electrical supply to them.
The bus, commonly known as Multibus II (registered trademark of Intel Corporation) is one of the buses used most in the industry at present. Its architecture is structured around a main bus of the parallel type, standardized in accordance with IEEE (Institute of Electrical and Electronic Engineers) standard 1296. Since the Multibus II is of the backplane type, the physical structure of an information processing system architecture is accordingly in the form of a board holder rack that holds multiple logic boards; the various physical and mechanical characteristics of the board holder rack are defined by the above standard.
Among the peripherals, mass memories such as rotating magnetic disk memories, are important. Magnetic disk memories are physically separate from the boards forming the information processing system itself and are connected to it by interface cables of the SCSI/IPI type, standardized by the American National Standards Institute (ANSI). Magnetic disk memories are also widely used, because of their low cost per unit of memory capacity (cost per unit of capacity, for example per kilobyte or megabyte).
Nevertheless, these rotating magnetic disk memories have the following disadvantages: They include a large mechanical portion, requiring high precision, and consequently are expensive (requiring a head positioning device for reading information and a motor to make the disk rotate); they are relatively large in volume; and the energy expenditure necessary for the function of the mechanical parts is essentially not insignificant. Finally, the access time for the information is on the order of from 10 to several tens of milliseconds.
The current trend in the industrial development of mass memories is to attempt to obtain better access time and to substantially reduce their physical bulk. To this end, mass memories known as electronic memories or electronic disks, also known as solid state disks, have thus been developed, which use semiconductor memories. Their access time is much less than a millisecond, i.e., several tens of times shorter than the access time of the most powerful rotating disk memories, and they have no rotating parts. The unit memory capacity cost of such electronic memories is still high, about 20 times higher than that of magnetic disk memories. However, the unit memory capacity cost of electronic memories has a tendency to decrease much faster than that of rotating magnetic disk memories, and it is conceivable that the cost will be about the same within a few years.
Electronic disk memories of this kind are currently made by the following manufacturers: Imperial Technology, Inc., 831 South Douglas Street, E1 Segundo, Calif. (its product is known as Megaram); and National Advanced Systems (with its memory unit 7990). Their memory capacity is on the order of several tens of megabytes, and their access time is several tenths of a millisecond. Their output is extremely high, on the order of several megabits per second.
Turning now to FIG. 1, an electronic disk DEA of this kind, known in the prior art, as manufactured by one of the aforementioned companies, is shown.
Such an electronic disk is parallelepiped in shape. It includes a plurality of logic boards 1-5, of which only the board 3 is shown, to make FIGS. 1A and 1B simpler. These various logic boards are parallel to one another and include a plurality of columns of semiconductor memories, for instance of the RAM type. These columns, which are n in number, are identified respectively by the symbols R.sub.1 . . . R.sub.j, . . . R.sub.n (in FIG. 1B, n=4, and four columns R.sub.1, R.sub.2, R.sub.3, R.sub.4 are shown). The information is written inside the various RAMs comprising the various columns C.sub.1 -C.sub.n, such a way that the blocks of information are successively written at successive addresses, in a manner similar to that conventionally used to write data into RAM memories.
The electronic disk DEA also includes two connectors 6 and 7, visible in the rear portion of the box, enabling connection either with other electronic disks of the same type or with the peripheral controller associated with these various electronic disks, by way of interface cables 8 and 9. These cables 8 an 9 are cables of the SCSI/IPI type, standardized by ANSI. It can be seen that the electronic disks of the DEA type are connected to one another and to their controller by means of interface connections of the same type as those used for rotating disk memories. This analogy in terms of connection with that of rotating disk memories has led those skilled in the art to call the electronic memory units electronic disks, even though the physical shape of the electronic memories is clearly not comparable with the physical shape of rotating magnetic disks.
The information (data and addresses where these data are located within RAM) originating from or proceeding to the RAM memories is carried over a bus BDA. This bus actually comprises one data bus and one address bus (for simplification in FIG. 1B, both these buses have been represented by a single bus). The bus BDA is subdivided in terms of the RAM columns into two branches BDA.sub.1 supplying the column R.sub.1 and R.sub.2, and BDA.sub.2 supplying the columns R.sub.3 and R.sub.4.
It will be noted that the physical bulk of the electronic disks such as DEA is equivalent to that of the rotating magnetic disk memories. Moreover, although the access time to the information contained in any one of the boards of an electronic disk is on the order of several tenths of a millisecond, the fact that the information is sent from a controller to an electronic disk or vice versa via interface links of the type such as links 8 and 9 considerably increases the access time, by on the order of from 1 to 2 milliseconds.
The present invention makes it possible to overcome these disadvantages by providing that the memory plane or planes of the electronic disk function (boards such as 1-5) are no longer disposed at a distance from the controller and processors of the information processing system, as in a conventional rotating magnetic disk memory or electronic disk of the prior art type (passing through the same type of interface link), but instead actually inside a compact assembly that combines the memory planes of the electronic disk units, its controller or controllers, and some of the processors of the information processing system to which the subsystem including the electronic disk units belongs. This compact assembly preferably takes the form of a board holder rack.
According to the invention, the electronic mass memory unit, the various elements of which comprise a plurality of boards each embodied by printed circuits carrying a plurality of semiconductor memories connected to a peripheral controller of the information processing system to which the unit belongs, is characterized in that it includes at least one central processor and one electronic disk unit including a motherboard containing the controller of the unit and a plurality of daughter boards comprising as many memory planes connected to one another 2 by 2, the first of them being connected to the motherboard, the central processor and the electronic disk unit by way of its motherboard being connected to a first and a second bus of the parallel type assuring the transport of information and electrical energy among the boards connected to it, the unit thus being accessible both via its own controller and via the central processor by way of the two buses.
In a preferred embodiment of the invention, the electronic mass memory unit has the particular feature that the two buses are identical and parallel to one another and have a overlap zone between them, to which the motherboard of the electronic disk unit and the central processor are connected. Preferably, the overlap zone is divided into two parts, each capable of receiving at least one electronic disk unit, each of the two parts being supplied with electrical energy by a supply that is separate item the supply to the other part.
To increase the security of access to the information memorized in the electronic memory unit of the invention, this electronic memory unit includes at least one backup rotating disk memory unit connected to the controller of the electronic disk unit and the central processor. Embodied in this way, the memory unit of the invention comprises a subsystem that belongs to a larger information processing system.
Further characteristics and advantages of the present invention will become more apparent from the ensuing detailed description of exemplary embodiments, referring to the accompanying drawings.